libbinder: avoid BpBinder subclasses

Being able to modify sizeof(BpBinder) at our discretion is very
important in order to prevent additional heap allocations. No one tends
to subclass this, because they are always created by Parcel, but at the
same time, we don't need to expose this possibility.

Bug: 167966510
Test: build only
Change-Id: I30c813d3de91043c46d235448fce41bac8fe5621